Hypertrophic cardiomyopathy is associated with a high incidence of sudden death. Eight patients with symptomatic hypertrophic cardiomyopathy were studied by atrial and ventricular programmed electrical stimulation in order to correlate the results with symptoms; with arrhythmias documented by continuous 24-hour electrocardiographic monitoring; and with electrocardiographic; echocardiographic, and hemodynamic measurements. Atrial stimulation induced sustained atrial tachyarrhythmias in 4 of 6 patients not having sustained atrial fibrillation prior to the study. Only 2 of these 6 patients had sustained atrial tachyarrhythmias documented by continuous 24-hour electrocardiographic monitoring; Ventricular stimulation induced sustained ventricular flutter-fibrillation in 1 of 8 patients—the patient with the longest run of unsustained ventricular tachycardia prior to our study. Atrial stimulation better identified patients who could sustain atrial tachyarrhythmias than continuous 24-hour electrocardiographic monitoring and demonstrated the hemodynamic consequences. Ventricular stimulation identified the patient most vulnerable to sustained ventricular tachyarrhythmias. Results of atrial and ventricular programmed electrical stimulation may guide patient-management decisions in an attempt to reduce sudden death due to hypertrophic cardiomyopathy.